Electrochromic color display having different electrochromic materials

ABSTRACT

An electrochromic display comprises electrochrome pixels ( 10 ) which comprise at least a first electrochrome material (EL 1 ) and a second electrochrome material (EL 2 ) between two electrodes (E 1 , E 2 ). Each of the electrochrome materials (EL 1 , EL 2 ) has two stable states, in one state at a first voltage across the electrochrome pixel ( 10 ) the material is transparent, in the other state at a second voltage across the electrochrome pixel ( 10 ) the material absorbs a color and thus is colored. The material changes from the one state to the other state by applying the appropriate one of the first or the second voltage. The amount of change of the absorption of the color depends on the time the appropriate voltage is applied. The first electrochrome material (EL 1 ) changes from a transparent state to a color absorbing state for at least partly absorbing a first color when a pixel voltage (VP) across the electrochrome pixel has the first value (V 1 ). The first electrochrome material (EL 1 ) changes from the color absorbing state to the transparent state when the pixel voltage (VP) has a second value (V 2 ) which has a polarity opposite to the first value (V 1 ). The second electrochrome material (EL 2 ) changes from a transparent state to a color absorbing state for at least partly absorbing a second color different than the first color when the pixel voltage (VP) has a third value (V 3 ) which has an absolute value smaller than an absolute value of the first value (V 1 ). The second electro-chrome material (EL 2 ) changes from the color absorbing state to the transparent state when the pixel voltage (VP) has a fourth value (V 4 ) which has a polarity opposite to the third value (V 3 ). An absolute value of the fourth value (V 4 ) is smaller than an absolute value of the second value (V 2 ).

The invention relates to an electrochromic display, a driver circuit fordriving an electrochrome pixel of the electrochromic display, a displayapparatus comprising the electrochromic display and the driver circuit,and a method of driving an electrochrome pixel of the electrochromicdisplay.

U.S. Pat. No. 4,304,465 discloses an electrochromic display device whichhas a polymer film on the display electrode. In the writing step, thepolymer film on the display electrode is oxidized to a colored,non-transparent form. In the erasing step, the polymer film is reducedto the neutral transparent form. The known electrochromic display deviceis not able to show a multicolor picture.

It is an object of the invention to provide an electrochromic displaydevice which is able to generate a multicolor picture.

A first aspect of the invention provides an electrochromic display asclaimed in claim 1. A second aspect of the invention provides a drivercircuit for driving an electrochrome pixel of the electrochromic displayas claimed in claims 8 to 10. A third aspect of the invention provides adisplay apparatus comprising the electrochromic display and the drivercircuit, as claimed in claim 11. A fourth aspect of the inventionprovides a method of driving an electrochrome pixel of theelectrochromic display as claimed in claims 12 to 14. Advantageousembodiments are defined in the dependent claims.

The electrochromic display comprises electrochrome pixels which compriseat least a first electrochrome material and a second electrochromematerial between two electrodes. The optical state of the electrochromematerial depends on the voltage applied across the pixel. At a firstvoltage across the electrochrome pixel the material is transparent, at asecond voltage across the electrochrome pixel the material absorbs acolor and thus appears colored. The material changes from the one stateto the other state by applying the appropriate one of the first or thesecond voltage. The amount of change of the absorption of the colordepends on the time the appropriate voltage is applied.

The first electrochrome material changes from a transparent state to acolor absorbing state to at least partly absorb a first color if a pixelvoltage across the electrochrome pixel has the first value. The firstelectrochrome material changes from the color absorbing state to thetransparent state if the pixel voltage has a second value which has apolarity opposite to the first value.

The second electrochrome material changes from a transparent state to acolor absorbing state to at least partly absorb a second color differentthan the first color if the pixel voltage has a third value which has anabsolute value smaller than an absolute value of the first value. Thesecond electro-chrome material changes from the color absorbing state tothe transparent state if the pixel voltage has a fourth value which hasa polarity opposite to the third value. An absolute value of the fourthvalue is smaller than an absolute value of the second value.

If such monochromic electrochromes are used, gray scales are created bycontrolling the degree of coloration by limiting the amount of chargeinjected in the electrochromic layer. In principle it would be possibleto generate a multicolor display by stacking at least two suchelectrochromic panels with electrochromic layers having differentcolors. A full color display would be obtained by using threeelectrochromic panels with different colors (preferably CMY, C=cyano,M=magenta, and Y is yellow). However, this will requires that threepanels are stacked on top of each other, causing parallax problems andwhich drastically increase the price of the display. Each of the panelscomprises at least a substrate, a working electrode with electrochromicmaterial, an electrolyte, a counter electrode with counter reactioncapability, and a substrate. Thus each panel has its own drivingelectronics on one of the substrates which drive the pixel electrodes,this adds greatly to the complexity, reduces the brightness of thedisplay, and adds to costs.

In the display in accordance with the invention, the electrochromematerials require different voltage values to change state. This enablesto drive the pixels with a single set of electrodes only, while stillenabling to control the amount of absorption of the differentelectrochrome materials separately.

Such a color electrochromic display is easy to manufacture because thelayer(s) of electrochromes can be applied easily, for example by usingscreen-printing, ink-jet printing or coating techniques.

In an embodiment as defined in claim 2, the first electrochrome materialand the second electrochrome material are present in two separatelayers. The layers are stacked on top of each other between theelectrodes.

In an embodiment as defined in claim 3, the first electrochrome materialand the second electrochrome material are implemented as a one layermixture. An advantage of this approach is an improved homogeneity of theresponse.

In an embodiment as defined in claim 4, the one layer mixture isabsorbed on the nano-porous area of one of the electrodes. The electrodeconsists of a nano-porous conducting material, for example, nanostructured titanium di-oxide. The nano-structured layer may cover an ITOor a FTO electrode. This has the advantage that a highly improveddiffusion of counter-ions for charge compensation in the electrochromicswitching process, and simultaneously an enhanced electron transfer toand from the electrochromes is achieved, resulting in an improvement ofthe response time of the device. Despite the monolayer coverage of sucha nano-porous electrode, a sufficient optical density in the coloredstates is still ensured due to the very high surface area of thenano-porous electrode.

In an embodiment as defined in claim 5, only two differentelectrochromic materials corresponding to two different colors arepresent in the pixel, while a color filter is provided for the thirdcolor. This prevents that it is necessary to identify threeelectrochromic materials with different coloration voltages (voltagesrequired to change the material towards the color absorbing state) andbleaching voltages (voltages required to change the material towards thetransparent state) which are also not damaged by (short) exposure tohigher voltages.

For example, each of the two different electrochromic materials isprovided in a separate layer. In a predetermined pixel, one of theelectrochromic materials is able to absorb red (i.e. appears cyan), theother electrochromic material is able to absorb green (i.e. appearsmagenta), and the color filter absorbs blue (i.e. appears yellow). Afull color matrix display is obtained by alternating the colorcombinations of different pixels. For example, in a pixel adjacent tothe predetermined pixel, one of the electrochromic materials is able toabsorb red (i.e. appears cyan), the other electrochromic material isable to absorb blue (i.e. appears yellow), and the color filter absorbsgreen (i.e. appears magenta).

In another aspect of the invention as defined in one of the claims 8 to10, the driver circuit supplies pixel voltages across the pixel in anorder which enables to set the amount of absorption of each one of thetwo electrochromic materials separately. In the embodiment in accordancewith the invention as defined in claim 8, first all electrochromicmaterials are bleached (put in the transparent state) then a voltage isapplied which is able to change the absorption of all the electrochromicmaterials. This voltage is applied as long as required to obtain thedesired amount of coloration of the electrochromic material whichrequires the highest voltage to change from transparent state toabsorbing state. Then a voltage is applied able to bleach the otherelectrochromic material while the electrochromic material which requiresthe highest voltage is unaffected. And finally, a voltage is appliedable to change the absorption of the other electrochromic material whilethe electrochromic material which requires the highest voltage isunaffected. This voltage is applied as long as required to obtain thedesired amount of coloration of the other electrochromic material.

In the embodiment in accordance with the invention as defined in claim9, first all electrochromic materials are colored then a voltage isapplied which is able to change the absorption of all the electrochromicmaterials towards the transparent state.

In another aspect of the invention as defined in claim 10, the drivercircuit supplies pixel voltages across the pixel in an order whichenables to set the amount of absorption of each one of the twoelectrochromic materials separately. The pixel is driven based on thedifference in color of the existing information and the color ofsuccessive information to be displayed. First the difference is detectedbetween the present amount of coloration of the first electrochromematerial which requires the highest voltage to change state and therequired future amount of coloration. The appropriate voltage is appliedacross the pixel to change the coloration of the first electrochromematerial in the correct direction, directly. The coloration of thesecond electrochrome material will change together with the colorationof the first electrochrome material. The resulting coloration of thesecond electrochrome material is compared with the required colorationand a voltage is applied across the pixel to change the coloration ofthe second electrochrome material in the correct direction, directly.This way of driving increases the switching (addressing) speed andreduces the power dissipation and degradation.

The sequential way of driving as claimed in claim 8 has the drawbacksthat a lot of steps have to be performed for writing a pixel such thatit has the correct total amount of absorption and the correct color isreached. Further, the fact that several materials are successivelycolored and bleached, before being finally colored (or the other wayaround: first bleached and then colored as defined in claim 9) causesmore charge to be moved than is required in the driving scheme asclaimed in claim 10. This increases the power dissipation and reducesthe lifetime of the display as degradation will occur faster if morecharge is flowing.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows a block diagram of an electrochromic display and itsdriving circuit,

FIG. 2 shows the structure of an electrochrome pixel in accordance withthe invention,

FIG. 3 shows the behavior of the three different electrochromicmaterials for elucidating driving schemes of the electrochromic display,

FIG. 4 shows the structure of an electrochrome pixel in accordance withthe invention,

FIG. 5 shows an embodiment for driving an electrochromic pixel in anactive matrix display, and

FIG. 6 shows another embodiment for driving an electrochromic pixel inan active matrix display.

FIG. 1 shows a block diagram of an electrochromic display and itsdriving circuit. The electrochromic display 1 comprises a matrix ofelectrochrome pixels 10 (further also referred to as pixels) associatedwith intersections of row (or select) electrodes RE extending in the rowdirection and column (or data) electrodes CE extending in the columndirection. A row driver 3 supplies select voltages to the row electrodesRE, and a column driver 2 supplies data voltages to the columnelectrodes CE. A data processor 5 receives input video VI and suppliestiming information TI to a controller 4, and a data signal DA to acomparator 6. The timing information TI may indicate the fields andlines in the video signal VI. The comparator 6 supplies the data signalDA′ to the column driver 2. The comparator 6 is optional, if thecomparator 6 is omitted, the data signals DA′ and DA are equal. Thecontroller 4 supplies a first control signal TI1 to the row driver 3 anda second control signal TI2 to the column driver 2. The timinginformation TI and the control signals TI1 and TI2 control the propersequence of voltages supplied to the electrochrome pixels 10, dependingon the desired driving scheme.

For a passive matrix display, the function of the row and columnelectrodes RE, CE and the row and column drivers 3, 2 may be exchanged,such that the row electrodes extend in the column direction.

The row driver 3 receives a power supply voltage VB1, and the columndriver 2 receives a power supply voltage VB2.

FIG. 2 shows the structure of an electrochrome pixel 10 in accordancewith the invention. The pixel 10 comprises from top to bottom: atransparent layer TL, a first electrode E1 which is part of the rowelectrode RE, a third electrochromic layer EL3, a second electrochromiclayer EL2, a first electrochromic layer EL1, a second electrode E2 whichis part of the column electrode CE, and a substrate SU. The pixelvoltage VP supplied by the row and column drivers 2, 3 between the firstand the second electrodes E1, E2 is shown as a voltage source VP. Inaddition, the pixel structure may further comprise an electrolyte layerto further assist the coloration process.

In a practical implementation, the pixel 10 comprises an electrolyte.This electrolyte may be present as a separate layer stacked within thecell. The electrolyte is deposited between the stack of electrochromesor mixture of electrochromes and the counter electrode E1. Furthermore,the counter electrode E1 maybe redox-active, or a separate redox-activelayer is present between counter electrode E1 and the elektrolyte layer,or a combination of both may be present.

FIG. 3 only shows one pixel element. Next to this pixel element otherpixels are present. The substrate will therefore not be limited to onlyone pixel, however, the color filter and the electrochromic layersshould be pixelated and be physically separated from neighboring pixels.The electrolyte however might extend laterally over the entire display.The counter electrode might be one common electrode or also pixelated.

The three electrochrome layers EL1, EL2 and EL3 may correspond in anyorder with materials showing a yellow, magenta or cyan coloration,respectively. Instead of the three electrochrome layers EL1, EL2 andEL3, it is also possible to mix the materials in a single layer. Thus,because the materials used is the important issue, and not whether thesematerials are divided over three layers or combined in two or even asingle layer is relevant to the invention. Therefore, the indices EL1,EL2 and EL3 are further used to indicate the materials. If the materialsare divided in three layers, these indices refer to the layers also.Although the three layers EL1, EL2 and EL3 enable a full color display,two layers suffice to make a display able to produce information withdifferent colors. Again the different materials in the two layers may bemixed in a single layer.

FIG. 3 shows the behavior of the three different electrochromicmaterials for elucidating driving schemes of the electrochromic display.The horizontal axis indicates the voltage VP across the electrochromematerial and the vertical axis indicates the amount of coloration of theelectrochrome material.

FIG. 3 concerns a full color electrochrome pixel 10 which comprisesthree different electrochrome materials EL1, EL2, EL3 in three separatelayers placed on a white reflecting substrate SU. Each of the threeelectrochrome materials EL1, EL2, EL3 switches between a fullytransparent state and a state that absorbs either red or green or bluelight while being transparent for the other two colors. The potentialrequired for this transition varies per electrochrome material EL1, EL2,EL3.

FIG. 3 shows the pixel voltage VP along the horizontal axis. In thevertical direction, the different electrochrome material EL1, EL2, EL3are shown. The white areas indicate for each color the area of voltageswherein the absorption state of the color does not (or only very slowly)change, the dashed areas indicate the voltages which cause theabsorption state to change. In this example, the material increasesabsorption when a voltage is applied within the right hand dashed partof the bars, and decreases absorption when a voltage is applied withinthe left hand dashed part of the bars. Dependent on the material used,this may be the other way around.

The first material EL1 does not change state (or changes state only veryslowly) if the pixel voltage VP supplied between the electrodes E1 andE2 is in the range from VL2 (which is a negative voltage) to VL1 asindicated by the non-dashed part of the bar indicated by EL1. The dashedpart of the bar EL1 for voltages lower than VL2 indicates that thecoloration of the layer EL1 decreases if a voltage V2 lower than VL2 isapplied. The amount of decrease depends on the time during which thevoltage V2 is supplied. The dashed part of the bar for voltages higherthan VL1 indicates that the coloration increases if a voltage V1 higherthan VL1 is applied. The amount of increase depends on the time duringwhich the voltage V1 is applied.

The second material EL2 does not change state (or changes state onlyvery slowly) if the pixel voltage VP supplied between the electrodes E1and E2 is in the range from VL4 to VL3 as indicated by the non-dashedpart of the bar indicated by EL2. The dashed part of the bar EL2 forvoltages lower than VL4 indicates that the coloration of the layer EL2decreases for voltages lower than VL4. The amount of decrease dependingon the time during which the voltage lower than VL4 is supplied. Thedashed part of the bar for voltages higher than VL3 indicates that thecoloration increases if a voltage is applied higher than VL3. The amountof increase depends on the time during which the voltage higher than VL3is supplied. Consequently, when the voltage V4 is applied to the cell10, the material EL2 will start bleaching while the state of thematerial EL1 will be substantially unaffected. In the same way, when thevoltage V3 is applied to the cell 10, the material EL2 will start toincrease the coloration while the state of the material EL1 issubstantially unaffected.

The third material EL3 does not change state (or changes state only veryslowly) if the pixel voltage VP supplied between the electrodes E1 andE2 is in the range from VL6 to VL5 as indicated by the non-dashed partof the bar indicated by EL3. The dashed part of the bar indicated by EL3for voltages lower than VL6 indicates that the coloration of the layerEL3 decreases if a voltage lower than VL6 is applied. The amount ofdecrease depending on the time during which the voltage lower than VL6is supplied. The dashed part of the bar for voltages higher than VL5indicates that the coloration increases if a voltage higher than VL5 isapplied. The amount of increase depends on the time during which thevoltage higher than VL5 is supplied. Consequently, when the voltage V6is applied to the cell 10, the material EL3 will start bleaching whilethe state of the other materials EL1 and EL2 will be substantiallyunaffected. In the same way, when the voltage V5 is applied to the cell10, the material EL3 will start to increase the coloration while thestate of the other materials EL1 and EL2 is substantially unaffected.

In this pixel 10 all the materials (or layers, if three layers arepresent) EL1, EL2, EL3 can be given any level of coloration by using thefollowing drive scheme.

Firstly, a voltage V2 which is lower than the voltage VL2 is suppliedbetween the electrodes E1 and E2 of the pixel 10 during a period of timelong enough to make all the layers EL1, EL2, EL3 transparent (the layersare bleached).

Secondly, the voltage V1 which is higher than the voltage VL1 issupplied between the electrodes E1 and E2. All the layers EL1, EL2, EL3start to color. The voltage V1 is removed at the instant the first layerEL1 has reached the desired absorption value.

Thirdly, the voltage V4 is applied in the range between VL2 and VL4causing the second and third layers EL2 and EL3 to bleach while thefirst layer EL1 is unaffected.

Fourthly, the voltage V3 in the range from VL3 to VL1 is applied, thefirst layer EL1 remains unaffected, while the second and the thirdlayers EL2 and EL3 start to color. The voltage V3 is removed at theinstant the second layer EL2 has reached the desired absorption value.

In a fifth step, the voltage V6 is applied in the range between VL4 andVL6, the third layer EL3 is bleached while the first and the secondlayers EL1 and EL2 are unaffected. In a sixth step, a voltage V5 in therange from VL5 to VL3 is applied, the first and second layers EL1 and EL2 remain unaffected, while the third layer EL3 starts to color. Thevoltage V5 is removed at the instant the third layer EL3 has reached thedesired absorption value.

At this point, all the layers EL1, EL2 and EL3 have reached theirdesired amount of coloration.

It is possible to change the order of the bleaching and colorizingsteps.

Although this drive scheme is able to drive the electrochromic display 1with the specially selected electrochromic materials EL1, EL2 and EL3 todisplay full color images, this sequential addressing approach due tothe many steps which have to be performed is relatively slow in writingan image. In addition, several materials EL1, EL2 and EL3 aresuccessively bleached and colored several times before the finalcoloration is reached. Consequently, a lot of charge is moved inaddressing a pixel 10 causing an increased power dissipation, and afaster degradation of the material EL1, EL2 and EL3.

A method of addressing in which the addressing speed is increased andthe power dissipation and degradation is reduced, drives the pixels 10such that the materials EL1, EL2 and EL3, in a first step, starting fromthe existing amount of coloration, are either bleached or colored asmuch as required to cause the desired new coloration of the pixel 10.

This drive scheme when applied to the construction of the pixel 10 asshown in FIG. 2 successively performs next steps:

In a first step, the current coloration of the first layer EL1 iscompared by the comparator 6 with the required coloration in thesuccessive new image. If the new coloration is more than the currentcoloration, the voltage V1 is supplied to the pixel 10. If the newcoloration is less than the current coloration, the voltage V2 issupplied to the pixel 10. Additional electrical circuitry in the pixelof an active matrix display (such as additional TFTs) may be required tocarry out the simultaneous application of one or the other voltage tothe pixel. All layers EL1, EL2 and EL3 start to change color. Thevoltage V1 or V2 is removed at the instant the first layer EL1 hasreached its desired new absorption value.

In a second step, the current coloration of the second layer EL2 iscompared by the comparator 6 with the required coloration in thesuccessive new image. If the new coloration is more than the currentcoloration (including the action of V1 or V2), the voltage V3 issupplied to the pixel 10. If the new coloration is less than the currentcoloration, the voltage V4 is supplied to the pixel 10. The layers EL2and EL3 start to change color, the first layer EL1 is unaffected. Thevoltage V3 or V4 is removed at the instant the second layer EL2 hasreached its desired new absorption value.

In a third and last step, the current coloration of the third layer EL3is compared by the comparator 6 with the required coloration in thesuccessive new image. If the new coloration is more than the currentcoloration (including the action of V1, V2, V3 or V4), the voltage V5 issupplied to the pixel 10. If the new coloration is less than the currentcoloration, the voltage V6 is supplied to the pixel 10. The third layerEL3 starts to change color, the first and second layers EL1 and EL2 areunaffected. The voltage V5 or V6 is removed at the instant the thirdlayer EL3 has reached its desired new absorption value.

In this way, only three voltage cycles have to be applied, reducing theaddressing time and the power dissipation. In general, the above drivingscheme applies to any cell 10 which contains three electrochromicmaterials EL1, EL2, EL3, it is not relevant that these materials arepresent in three layers as is shown in FIG. 2. Thus, in general, theterm layer(s) may be replaced by material(s).

By way of example, a material which has the behavior shown in FIG. 3 isdescribed now. In a test pixel (electrochromic cell) 10, a layer of 300nanometer thick PEDOT is spin-coated onto an ITO/glass substrate whichis used as a working electrode E2. A pixel 10 is constructed by gluingthis substrate to a further ITO/glass substrate which is used as thecounter electrode E1. A cell 10 gap between these two electrode layersE1 and E2 is filled with an electrolyte solution containing 0.2 M LiClO₄(lithium perchlorate) in K-butyrolactone. The cell 10 is colored byapplying a voltage of 3 volts across it, which causes a rapid bluecoloration of the PEDOT layer by a reduction reaction of the PEDOT. Thecell 10 starts to bleach slowly at a voltage of −1 volts across it. Forvoltages between −0.5 and 2.5 volts the color of the cell changes hardlyin time. At −1.5 volts a fast bleaching occurs, and after some time thePEDOT is oxidized to its conducting and almost transparent state.

The above drive schemes are related to a full color display with threedifferent electrochromic materials. These drive schemes, in a simplifiedversion, by leaving out one cycle, can also be used to drive a colordisplay with two different electrochromic materials. Such a display canonly display colors caused by mixing the two colors corresponding to thetwo materials.

FIG. 4 shows the structure of an electrochrome pixel in accordance withthe invention. This pixel 10 comprises from top to bottom: a colorfilter CF, a first electrode E1 (the reference electrode), a firstelectrochromic layer EL1, a second electrochromic layer EL2, a firstelectrochromic layer EL1, a second electrode E2 (the pixel electrode),and a substrate SU. The substrate may also comprise TFTs and otherelectronic components (not illustrated). The pixel voltage VP suppliedby the row and column drivers 2, 3 between the first and the secondelectrodes E1, E2 is shown as a voltage source VP. In addition, thepixel structure may further comprise an electrolyte layer to furtherassist the coloration process.

The two electrochrome layers EL1 and EL2 may correspond in any orderwith materials showing a yellow, magenta or cyan coloration,respectively. Instead of the two electrochrome layers EL1 and EL2, it isalso possible to mix the materials in a single layer. The color of thecolor filter CF has to be selected as the complementary color of thecolors of the two electrochrome layers EL1 and EL2. If, for example, thecolor of the electrochrome layers EL1 and EL2 is cyan and magenta, thecolor filter CF should be yellow. By alternating the color combinationsof the layers EL1 and EL2 and the color filter CF for adjacent pixels10, it is possible to provide a color display. Because only two insteadof three different electrochrome materials EL1 and EL2 have to beaddressed, more materials can be selected which have the differentvoltage levels for bleaching and coloration.

In the same manner as elucidated with respect to FIG. 2, only one cellis shown, and the electrolyte is not shown.

The color filter CF is preferably located as close as possible to theelectrochrome materials EL1 and EL2.

FIG. 5 shows an embodiment for driving an electrochromic pixel in anactive matrix display.

The electrochromic display 1 has an active matrix structure, whereineach pixel 10 comprises thin film transistors (further referred to asTFT) TR1 and TR2 in order to drive the pixel 10. The main current pathof the drive TFT TR1 is arranged between the pixel electrode E1 of thepixel 10 and a power line voltage VB. The common electrode E2 of thepixel 10 is connected to ground. The main current path of the addressingTFT TR2 is connected between a column electrode CE and the controlelectrode of the drive TFT TR1. The control electrode of the addressingTFT TR2 is connected to a select electrode RE.

The select voltages on the rows RE are used to address a row RE ofpixels 10 by activating the addressing TFT TR2 to conduct. The datavoltage from the column CE is then passed to the control electrode ofthe drive TFT TR1 and determines whether this TFT is conducting, ornon-conducting. The drive TFT TR1 connects the pixel electrode E1 to apower supply line on which the power supply voltage VB is present. Thedata voltage therefore determines whether the pixel 10 is attached(pixel is driven) or not attached (pixel is not driven) to the powersupply voltage VB. A memory element in the pixel circuit (for example astorage capacitor CS) ensures that the pixel 10 remains driven until thenext addressing period, one frame time later. At this point, the powersupply voltage VB can be changed to supply a different one of thevoltages V1 to V6 to the pixel 10.

An electrochrome layer EL1, EL2, EL3 can be colored and bleached in thefollowing steps:

-   -   (i) The power supply voltage VB is switched to the bleaching        voltage, and all pixels 10 are addressed with a high voltage,        whereby all pixels 10 are bleached (pixels which are already        bleached will do nothing at this stage). The storage capacitor        CS ensures that the drive TFT TR1 remains conducting during the        hold period.    -   (ii) All pixels 10 are addressed with a low voltage. This turns        the drive TFTs TR1 off. The power supply voltage VB is switched        to the coloring voltage.    -   (iii) Those pixels 10 in the row selected by the row select        voltage on the row RE which require coloring are addressed to a        high voltage by a high data voltage on the data electrode CE.        The drive TFT TR1 becomes conductive and coloration begins. The        storage capacitor CS ensures that the drive TFT TR1 remains        conducting during the hold period. When the pixel 10 is        sufficiently colored, the pixel 10 is disconnected from the        power line by addressing the pixel 10 with a low voltage. When        the new image is written, the power supply voltage VB can be        powered down.

In this embodiment, the grey level (“intensity”) of the color will bedefined by the integral amount of charge passing into the electrochromelayer EL1, EL2, EL3 and hence by the time in which the pixel electrodeE1 is connected to the power line.

FIG. 6 shows another embodiment for driving an electrochromic pixel inan active matrix display. In FIG. 6, a more complex pixel circuit isshown whereby an electrochrome layer EL1, EL2, EL3 can be colored andbleached.

A pixel 10 has a pixel electrode E1 and a common electrode E2 connectedto ground. A series arrangement of main current paths of two drive TFTsTR12 and TR13 is arranged between a power supply voltage VB1 and a powersupply voltage VB2. The junction of the two drive TFTs TR12 and TR13 isconnected to the pixel electrode E1.

A main current path of an address TFT TR10 is arranged between a columnelectrode CE to receive the column data CD1 and the control electrode ofthe drive TFT TR12. The control electrode of the address TFT TR10 isconnected to a select electrode RE to receive a row select signal RS1. Astorage capacitor CH1 is connected to the control electrode of the driveFET TR12.

A main current path of an address TFT TR11 is arranged between a columnelectrode CE to receive the column data CD2 and the control electrode ofthe drive TFT TR13. The control electrode of the address TFT TR11 isconnected to a select electrode RE to receive a row select signal RS2. Astorage capacitor CH2 is connected to the control electrode of the driveFET TR12.

The operation of the pixel circuit is elucidated in the now following.The power supply voltages VB1 and VB2 are set to a bleaching voltage andcoloration voltage, respectively. The display is addressed with twovoltages: a high voltage causes the drive TFT TR12, TR13 to becomeconductive, a low voltage stops the conducting state of the drive TFTTR12, TR13. The column data CD1 is used to select pixels 10 whichrequire coloring, and the column data CD2 is used to select pixels 10which require bleaching. Those pixels 10 which require coloring orbleaching are addressed to a high voltage. The drive TFTs TR12, TR13become conducting and bleaching or coloration starts. The storagecapacitors CH1, CH2 ensure that the drive TFTs TR12, TR13 remainconducting during the hold period. When the pixel 10 is sufficientlycolored or bleached, the pixel 10 is disconnected from the power supplyvoltage VB1, VB2 by addressing the pixel 10 with a low voltage. When thenew image is written, the power supply voltages VB1 and VB2 can bepowered down.

The addressing of the a pixel 10 is performed by the row select signalsRS1 and RS2 and the column data CD1 and CD2.

Again, in this embodiment, the grey level (“intensity”) of the colorwill be defined by the integral amount of charge passing into theelectrochrome layer EL1, EL2, EL3 and hence by the time in which thepixel electrode E1 is connected to the power supply voltages VB1, VB2.As in general no “reset” will be used, it is necessary to know theprevious state of the pixel 10 before supplying the correct amount ofcharge (or discharge) to reach the new grey level. This will require asignal processing approach, wherein the previous grey level is stored ina frame memory, the new grey level is compared with the previous greylevel, the required charge determined (via a look-up-table or analyticalfunction), and the desired pixel data is supplied to the pixel 10.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims.

For example, it would also be possible to use electrodes which generatein-plane fields in combination with the driving approach in accordancewith embodiments of the invention. In this way area defined gray scalescould be generated for different colors, which could also be used incombination with red, green, or blue electrochromic layers.

The display can be operated either in a transmissive setup, e.g. bylighting the device with a backlight system, but is more likely to beused in reflective setup, e.g. by using a reflector (preferably diffuse)behind the display.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word “comprising” does notexclude the presence of elements or steps other than those listed in aclaim. The invention can be implemented by means of hardware comprisingseveral distinct elements, and by means of a suitably programmedcomputer. In the device claim enumerating several means, several ofthese means can be embodied by one and the same item of hardware. Themere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

1. An electrochromic display comprising electrochrome pixels comprisingat least a first electrochrome material and a second electrochromematerial between two electrodes, the first electrochrome materialchanging from a transparent state to a color absorbing state for atleast partly absorbing a first color when a pixel voltage across theelectrochrome pixel has a first value, the first electrochrome materialchanging from the color absorbing state to the transparent state whenthe pixel voltage has a second value having a polarity opposite to thefirst value, and the second electrochrome material changing from atransparent state to a color absorbing state for at least partlyabsorbing a second color different than the first color when the pixelvoltage has a third value having an absolute value being smaller than anabsolute value of the first value, the second electro-chrome materialchanging from the color absorbing state to the transparent state whenthe pixel voltage has a fourth value having a polarity opposite to thethird value, an absolute value of the fourth value being smaller than anabsolute value of the second value.
 2. An electrochromic display asclaimed in claim 1, wherein the first electro-chrome material and thesecond electrochrome material are two separate layers.
 3. Anelectrochromic display as claimed in claim 1, wherein the firstelectro-chrome material and the second electrochrome material are mixedin a one layer mixture.
 4. An electrochromic display as claimed inclaims 2 or 3, wherein one of the electrodes has a nano-porous surfacebeing covered by the one layer mixture.
 5. An electrochromic display asclaimed in claim 1, wherein the electrochrome pixels comprise a colorfilter for filtering a third color being different than the first colorand the second color.
 6. An electrochromic display as claimed in claim1, wherein the electrochrome pixels further comprise a thirdelectrochrome material changing from a transparent state to a colorabsorbing state for at least partly absorbing a third color differentthan the first and the second color when the pixel voltage has a fifthvalue having an absolute value being smaller than an absolute value ofthe third value, the third electro-chrome material changing from thecolor absorbing state to the transparent state when the pixel voltagehas a sixth value having a polarity opposite to the third value, anabsolute value of the sixth value being smaller than an absolute valueof the fourth value.
 7. An electrochromic display as claimed in claim 6,wherein the first, second a third electrochrome material in their colorabsorbing state appear cyano, magenta, and yellow, respectively.
 8. Adriver circuit for driving an electrochrome pixel of the electrochromicdisplay as claimed in claim 1, the driver circuit comprising means forapplying the pixel voltage across the electrochrome pixel successivelyas follows: (i) the pixel voltage has an absolute value and a polarityfor changing towards the transparent state of both the firstelectrochrome material and the second electrochrome material, (ii) thepixel voltage has an absolute value and a polarity for changing thetransparent state into the color absorbing state of both the firstelectrochrome material and the second electrochrome material, and isapplied as long as required to obtain a desired amount of absorption ofthe first electrochrome material, (iii) the pixel voltage has anabsolute value and a polarity for changing towards the transparent stateof the second electrochrome material, while the first electrochromematerial is unaffected, and (iv) the pixel voltage has an absolute valueand a polarity for changing the transparent state of the secondelectrochrome material into the color absorbing state, while the firstelectrochrome material is unaffected, and is applied as long as requiredto obtain a desired amount of absorption of the second electrochromematerial.
 9. A driver circuit for driving an electrochrome pixel of theelectrochromic display as claimed in claim 1, the driver circuitcomprising means for applying the pixel voltage across the electrochromepixel successively as follows: (i) the pixel voltage has an absolutevalue and a polarity for changing towards the color absorbing state ofboth the first electrochrome material and the second electrochromematerial, (ii) the pixel voltage has an absolute value and a polarityfor changing the color absorbing state into the transparent state ofboth the first electrochrome material and the second electrochromematerial, and is applied as long as required to obtain a desired amountof absorption of the first electrochrome material, (iii) the pixelvoltage has an absolute value and a polarity for changing towards thecolor absorbing state of the second electrochrome material, while thefirst electrochrome material is unaffected, and (iv) the pixel voltagehas an absolute value and a polarity for changing the color absorbingstate of the second electrochrome material into the transparent state,while the first electrochrome material is unaffected, and is applied aslong as required to obtain a desired amount of absorption of the secondelectrochrome material.
 10. A driver circuit for driving anelectrochrome pixel of the electrochromic display as claimed in claim 1,the driver circuit comprising a comparator for comparing a currentamount of absorption of the first electrochrome material with a requiredamount of absorption required for successive information to bedisplayed, means for applying the pixel voltage across the electrochromepixel having an absolute value and a polarity for changing towards thetransparent state of both the first electrochrome material and thesecond electrochrome material, when the required amount of absorption islower than the current amount of absorption, or for applying the pixelvoltage across the electrochrome pixel having an absolute value and apolarity for changing towards the color absorbing state of both thefirst electrochrome material and the second electrochrome material, whenthe required amount of absorption is higher than the current amount ofabsorption, the comparator being adapted for comparing a current amountof absorption of the second electrochrome material with a requiredamount of absorption required for successive information to bedisplayed, the means for applying the pixel voltage being adapted forsupplying the pixel voltage across the electrochrome pixel having anabsolute value and a polarity for changing towards the transparent stateof the second electrochrome material while the first electrochromematerial is unaffected, when the required amount of absorption is lowerthan the current amount of absorption, or for applying the pixel voltageacross the electrochrome pixel having an absolute value and a polarityfor changing towards the color absorbing state of the secondelectrochrome material while the first electrochrome material isunaffected, when the required amount of absorption is higher than thecurrent amount of absorption.
 11. A display apparatus comprising thecolor electrochromic display as claimed in claim 1, and the drivercircuit as claimed in any of the claim 8 to
 10. 12. A method of drivingan electrochrome pixel of the electrochromic display as claimed in claim1, comprising applying the pixel voltage across the electrochrome pixelsuccessively as follows: (i) the pixel voltage has an absolute value anda polarity for obtaining the transparent state of both the firstelectrochrome material and the second electrochrome material, (ii) thepixel voltage has an absolute value and a polarity for changing thetransparent state into the color absorbing state of both the firstelectrochrome material and the second electrochrome material, and isapplied as long as required to obtain a desired amount of absorption ofthe first electrochrome material, (iii) the pixel voltage has anabsolute value an a polarity for obtaining the transparent state of thesecond electrochrome material, while the first electrochrome material isunaffected, and (iv) the pixel voltage has an absolute value and apolarity for changing the transparent state of the second electrochromematerial into the color absorbing state, while the first electrochromematerial is unaffected, and is applied as long as required to obtain adesired amount of absorption of the second electrochrome material.
 13. Amethod of driving an electrochrome pixel of the electrochromic displayas claimed in claim 1, comprising applying the pixel voltage across theelectrochrome pixel successively as follows: (i) the pixel voltage hasan absolute value and a polarity for obtaining the color absorbing stateof both the first electrochrome material and the second electrochromematerial, (ii) the pixel voltage has an absolute value and a polarityfor changing the color absorbing state into the transparent state ofboth the first electrochrome material and the second electrochromematerial, and is applied as long as required to obtain a desired amountof absorption of the first electrochrome material, (iii) the pixelvoltage has an absolute value an a polarity for obtaining the colorabsorbing state of the second electrochrome material, while the firstelectrochrome material is unaffected, and (iv) the pixel voltage has anabsolute value and a polarity for changing the color absorbing state ofthe second electrochrome material into the transparent state, while thefirst electrochrome material is unaffected, and is applied as long asrequired to obtain a desired amount of absorption of the secondelectrochrome material.
 14. A method of driving an electrochrome pixelof the electrochromic display as claimed in claim 1, comprisingcomparing a current amount of absorption of the first electrochromematerial with a required amount of absorption required in for successiveinformation to be displayed, applying the pixel voltage across theelectrochrome pixel having an absolute value and a polarity for changingtowards the transparent state of both the first electrochrome materialand the second electrochrome material, when the required amount ofabsorption is lower than the current amount of absorption, or forapplying the pixel voltage across the electrochrome pixel having anabsolute value and a polarity for changing towards the color absorbingstate of both the first electrochrome material and the secondelectrochrome material, when the required amount of absorption is higherthan the current amount of absorption, comparing a current amount ofabsorption of the second electrochrome material with a required amountof absorption required for successive information to be displayed, andapplying the pixel voltage being adapted for supplying the pixel voltageacross the electrochrome pixel having an absolute value and a polarityfor changing towards the transparent state of the second electrochromematerial while the first electrochrome material is unaffected, when therequired amount of absorption is lower than the current amount ofabsorption, or for applying the pixel voltage across the electrochromepixel having an absolute value and a polarity for changing towards thecolor absorbing state of the second electrochrome material while thefirst electrochrome material is unaffected, when the required amount ofabsorption is higher than the current amount of absorption.